1. Field of the Invention
The present invention relates to a technology for suppressing the reproduction error of a transmission signal, and more particularly relates a technology for suppressing the reproduction error of a transmission signal in high-speed communication.
2. Description of the Related Art
Lately the transfer rate of data in a communication system has remarkably increased, and in a transmission line using an optical fiber, a transfer rate of approximately 10 Gb/s has been realized.
Generally, when receiving an optical signal transmitted from the optical transmitting device of the communication system, the receiving device of a communication system, such as the optical receiving device of an optical communication system using the above-mentioned optical fiber as a transmission medium converts the optical transmission signal into an electrical signal, reshapes the waveform of the electrical signal by a built-in equalizer and identifies data put on the output signal.
This data is identified by discrimination circuit built in the receiving device, based on both discrimination phase that is determined using a clock signal extracted from the output signal and a threshold voltage of “0” and “1” (hereinafter called “discrimination level or “discrimination voltage”, and a set of the discrimination phase and discrimination level is called “discrimination point”). Thus, the data put on the output signal is reproduced.
However, the optical transmission waveform of an optical transmission signal that travels through the above-mentioned optical fiber deforms by the influences of noise, the transmission characteristic of an optical fiber and the like before the signal reaches the receiving device. Therefore, if the signal is reproduced by the built-in discrimination circuit in which a discrimination point is preset, in the optical receiving device receiving the signal, there occurs a deviation between the discrimination point and the optimal discrimination point of this case, and reproduction errors continue to occur.
As a method for visualizing this waveform deformation phenomenon, a method for displaying an eye pattern is known. This eye pattern can be formed by repeatedly overlapping a signal synchronous with a clock signal timewise using a measurement instrument, such as oscilloscope on the like. If a signal is outputted from the above-mentioned equalizer circuit, an eye pattern in which “mark” and “space” are overlapped is formed on a display screen in which time and current/voltage are taken on the horizontal and vertical axes, respectively, by repeatedly overlapping the output signal synchronous with a clock signal extracted the output signal timewise. Thus, coordinates (discrimination point) to be determined by both a predetermined value (discrimination phase) on the time axis and a predetermined value (discrimination level) on the current/voltage axis can be plotted on the eye pattern.
If waveform deformation occurs, the outline of the eye pattern becomes thick a little (shape obtained by overlapping a continuous signal waveform timewise), and a boundary between adjacent signal waveforms becomes vague. As a result, an eye opening becomes narrow and a valid discrimination area (range of discrimination points where “1” and “0” can be satisfactorily discriminated) also becomes narrow. Although there are a variety of waveform deformations, for example, sometimes the top of a waveform deforms and sometimes its bottom deforms, the allowable valid range (phase margin) of a discrimination phase in the time-axis direction for each unit in the current/voltage-axis direction on the eye pattern varies as time elapses. Usually, the phase margin is maximized at a cross-point between the “mark” waveform of the eye pattern and its “space” waveform, and the phase margin decreases as the signal shifts toward the top or bottom of the waveform using the point as a center. An optimal point where a discrimination error is hardest to occur in the valid discrimination area also shifts depending on the type of waveform deformation. This shift of an optimal point similarly occurs if a transmitted signal shifts in the current/voltage-axis direction.
As described above, if a waveform deformation phenomenon occurs, a point on the current/voltage axis where the phase margin is maximized is shifted in the current/voltage-axis direction. Therefore, in order to satisfactorily discriminate “1” from “0”, it is important to detect an optimal discrimination level where the phase margin is maximized.
In order to cope with this waveform deformation, technologies for dynamically shifting the discrimination point to an optimal point are disclosed.
In one of such technologies, two discrimination circuits in which the discrimination point is deviated vertically (in the current/voltage-axis direction) or horizontally (in the time-axis direction) from the reference position are provided against a discrimination circuit with the reference discrimination point, and the reference discrimination point (discrimination level in the current/voltage-axis direction or discrimination phase in the time-axis direction) is independently shifted to an optimal position, based on the respective operation results of the reference discrimination circuit and each of the discrimination circuits. Especially, when adjusting the discrimination level to an optimal position, a common discrimination phase is given to each discrimination circuit by inputting the same clock signal to all the discrimination circuits, and the discrimination level is optimized on the same conditions in the time-axis direction. When adjusting the discrimination phase to an optimal position, a common discrimination level is given to all the discrimination circuits, and the discrimination phase is optimized on the same conditions in the current/voltage-axis direction. In this case, each discrimination circuit obtains a result using a clock signal with the reference phase that is based on a clock signal made recovery from a timing recovery unit, a clock signal with a phase advanced from the reference phase and a clock signal with a phase delayed from the reference phase. Then, the phase of the clock signal is changed so that the discrimination phase of the reference discrimination circuit may come to an optimal position, based on these results (Japanese Patent Laid-open Application No. 8-265375).
In another of the technologies, both a discrimination circuit discriminating an input data signal using a clock signal with a specific reference phase and a discrimination circuit discriminating the input data signal using a clock signal phase-modulated using the reference phase as the center are provided, the phase of the clock signal is changed based on the respective operation results of the outputs of these discrimination circuits and the discrimination point is shifted to an optimal point (Japanese Patent Laid-open Application No. 11-68674).
When reproducing signals transmitted from the transmitting device in the receiving device, a waveform deformation phenomenon in a transmission route is conventionally a problem.
Among a variety of communication systems, in the case of an optical communication system for transmitting optical transmission signals using the above-mentioned optical fiber as a transmission medium, optical transmission waveform deformation by the influences of noise, the transmission characteristic of an optical fiber and the like remarkably appears and greatly affects the reproduction of optical transmission signals. More particularly, in a high-speed optical communication system whose transfer rate exceeds, for example, 40 Gb/s, optical transmission waveform degrades by the influences of the dispersion characteristic/non-linearity of an optical fiber, the lack of a high-speed characteristic of an optical receiver circuit or the like, which raises a critical problem in the signal reproduction of an optical receiving device.
When solving the deviation problem of an optimal discrimination point due to this waveform degradation by the prior art, the highly accurate phase matching technology of a plurality of branched clock signals and a phase changing circuit for changing the phase of a clock signal with high accuracy are needed.
Nowadays, the promotion of integrated circuits is desired for the purpose of miniaturization. For example, if the transfer rate of a data signal is 40 Gb/s, one timeslot is 25 ps and is very short. Therefore, according to the prior art, phase matching with accuracy in order of 1 ps is needed.
In order to receive transmission signals with less waveform deformation in a receiving device, further the stabilization of transmission signals transmitted from a transmitting device is studied.